1. Field of the Invention
This invention relates to the fabrication of lightweight plastic transparencies, particularly those comprising at least one sheet of acrylic plastic resin. The acrylic resin may form a monolithic sheet or it may be laminated to additional sheets to form a laminated transparency especially suitable for aircraft.
The transparencies for aircraft are required to fit within a frame and have to be shaped to fit within the frame. In addition, it is desirable for good optics that the surfaces of the acrylic resin sheet that forms an exterior or interior surface of an aircraft transparency be as free from surface marks as possible.
Acrylic resin is used as a transparent shielding device where glass is unsafe, too heavy, or otherwise impractical. Cast polymethyl methacrylate, which is currently marketed under such trademarks as PLEXIGLAS by Rohm and Haas and LUCITE by DuPont, is a typical acrylic resin that has several characteristics which make it ideally suited for aircraft transparencies, such as canopies and windows for lightweight aircraft. For example, it is considerably lighter than glass, yet is quite strong at ambient temperatures. Moreover, while acrylic resin is relatively soft compared to glass and therefore scratches more readily than glass, the scratches are easily removed merely by polishing the marred area with common polishing compounds. Furthermore, acrylic resin is fairly stable from a chemical standpoint. It is relatively unaffected by most solvents. Furthermore, acrylic resins accept ultraviolet light stabilizers without significantly impairing their light transmitting characteristics.
For the above reasons, acrylic resin and particularly cast polymethyl methacrylate has been used as an outer layer of either monolithic or laminated curved aircraft transparencies, particularly for lightweight planes which fly at relatively low altitudes and at relatively low speeds. Under such circumstances, the acrylic windows, windshields or canopies of aircraft do not develop a temperature above the softening point of the acrylic resin, which is in the vicinity of 225.degree. F. (107.degree. C.).
The optical properties of solid, acrylic resin sheet surfaces have been improved and surface marks reduced in intensity by press polishing. During press polishing, an acrylic resin sheet is held against a relatively rigid mold having an optically smooth surface to form an assembly. The assembly is subjected to sufficient elevated temperature and pressure for sufficient time for said acrylic resin sheet surface to develop the smoothness of the mold surface. The temperature at which pressure is applied is below the deformation temperature of the mold and sufficiently high to soften and press polish the surface of the solid acrylic resin sheet. However, unless the mold surface is composed of a material compatible with that of a solid acrylic resin sheet, defects are imparted to the surface of the solid acrylic resin sheet either during press polishing or during the separation of the mold from the press polished surface of the acrylic resin sheet. The mold materials suggested for use in the prior art for press polishing solid acrylic sheets have not been as good as desired for reasons mentioned in the discussion of specific prior art materials that follows.
In the past, acrylic resin sheets have been press polished by molds composed of various metals such as stainless steel coated with a high temperature mold release agent, or by a highly mirror polished plate of hard rolled brass that may be nickel plated or plated with coatings of nickel and chromium, for example, or by non-metallic pressing materials such as smoothly surfaced sheets of polyethylene glycol of terephthalic acid and smoothly surfaced glass plates provided on their resin facing surface with a suitable parting material.
It is also known to laminate an acrylic resin sheet to either another acrylic resin sheet or to a polycarbonate sheet through an interlayer material or directly to the polycarbonate sheet using an adhesive such as an epoxy resin by applying a highly polished rigid member against the surface to be press polished under sufficient heat and pressure to simultaneously laminate the layers together during press polishing.
Metal or glass press polishing or laminating molds have been found suitable for press polishing and laminating flat sheets of acrylic plastic resin. However, when the acrylic resin surface to be press polished is curved, metal press polishing molds cause the convex surface of the acrylic plastic sheet to develop tiny surface openings during press polishing even when the press polishing molds are provided with parting material. Glass and metal molds used to press polish shaped sheets of rigid plastic such as polycarbonate and acrylic resin are difficult to shape to complicated shapes. Metal molds must be ground and polished to develop smooth surfaces. Polishing marks in the shaped surface of the metal press polishing molds tend to print off onto the surface of a plastic sheet that is press polished. Furthermore, glass compositions tend to devitrify when heated to elevated temperatures required for shaping to complicated shapes. The devitrification pattern on the glass tends to print off onto the plastic surface, even with a thin layer of parting material applied to the glass press polishing mold.
Shaped molds covered with polyethylene glycol of terephthalic acid (Mylar) are difficult to make because it is hard to apply the cover to a shaped surface without causing wrinkling. Shaped press polishing molds for use with shaped acrylic plastic sheets that do not cause flaws during press polishing were needed prior to the present invention.